Dynamic spectrum management level-3 (DSM3) or “vectoring” is a technique in DSL communication systems for mitigating the crosstalk inherent in twisted-pair networks by cancelling or precoding the signals from a multiplicity of collocated transceivers. Certain aspects of vectoring that can be considered background art are described in U.S. Patent Publ. No. 2009/0245340, U.S. Patent Publ. 2008/0049855, U.S. Patent Publ. No. 2010/0195478, U.S. Patent Publ. No. 2009/0271550, U.S. Patent Publ. No. 2009/0310502, U.S. Patent Publ. No. 2010/0046684 and U.S. Pat. No. 7,843,949.
Among other things, the G.vector (G.993.5) standard provides a framework for actively cancelling far-end crosstalk (FEXT) among lines in the vectored DSL system. This framework provides for lines to non-disruptively join the vectored system by enabling estimation of coefficients for mitigating FEXT (a precoder for downstream transmissions and a post-canceller for upstream transmissions) from and into the initializing (i.e., joining) lines. This framework is created by introducing new phases (signaling as well as messaging) and modifying some of the existing phases of initialization provided by the G.993.2 (VDSL2) standard. The new signaling phases introduced by G. vector may be broadly partitioned into two groups. The first group is vector-1 signals. These consist of sync symbols only with intervening silence (i.e., the transmitter goes quiet between sync symbols). Additionally, a predefined binary pilot sequence modulates tones of the sync-symbols. The primary purpose of vector-1 signals is to enable mitigation coefficient estimation for FEXT from the joining line(s) into the lines that are already in Showtime. The second group is vector-2 signals. These consist of sync symbols modulated by a pilot sequence as well as regular symbols carrying the special-operations channel (SOC) messages. The primary purpose of vector-2 signals is to enable mitigation coefficient estimation for FEXT into the joining line(s) (from other joining lines as well as from lines that are already in Showtime).
A typical G.vector initialization involves six distinct and non-overlapping G.vector signaling phases: Four non-overlapping phases comprising 0-P-VECTOR 1, R-P-VECTOR 1, 0-P-VECTOR1-1 and R-P-VECTOR 1-1; One phase of overlapped 0-P-VECTOR 2 and R-P-VECTOR 1-2; and One phase of overlapped 0-P-VECTOR 2-1 and R-P-VECTOR 2.
Pilot sequences are provisioned in G.vector to enable the accurate estimation of FEXT mitigation coefficients between any pair of lines in the vectored system. The G.vector standard allows the vector control entity (VCE) to assign the pilot sequence to each line; however, it does not specify any details on the sequences that must be used (i.e. their choice, composition, etc. can be vendor-discretionary).
A common process uses a set of orthogonal sequences, wherein every user is assigned a unique sequence of length (or period) L. For a vectored system of N users, FEXT mitigation coefficients on a given tone (or sub-carrier) between any pair of users can be unambiguously resolved at the end of a pilot sequence period if L≧N. This suggests that the duration of the vector-1 or vector-2 phases must be at least L sync-symbols (with L≧N) to guarantee successful estimation of FEXT mitigation coefficients between any pair of users on a specific tone. In an exemplary vectored system with N=512 users, where the system uses the pilot sequence process above, each new G.vector signaling phase must last at least 512 sync symbols (approximately 32 seconds for a 4 kHz symbol-rate system). Thus, a typical G.vector initialization would require more than three minutes (6 phases×32 seconds) of additional time over and above the time required for G.993.2 initialization (about 40 seconds). Three or four minutes for initializing a G.vector line before entering Showtime is highly undesirable from a customer perspective, and most of this type of delay would be due to additional time spent in the G.vector signaling phases. The problem can be further exacerbated when one pilot sequence period is insufficient to achieve vectored signal-to-noise ratio (SNR) performance that is reasonably close to ideal FEXT-free SNR due to the impact of noise in estimates of the FEXT mitigation coefficients, meaning multiple pilot-sequence periods may have to be accommodated in the G.vector signaling phases.
Accordingly, methods and apparatuses for reducing the time needed for G.vector initialization are desirable.